Bega-Ch-6.Pdf

147 CHAPTER SIX STAGE THREE : THE FUTURE TRAJECTORY OF CHANGE AND RECOVERY POTENTIAL OF RIVERS IN BEGA CATCHMENT 6.1 Introduction and outline of this chapter The analysis of river recovery potential interprets the trajectory of likely future reach adjustments under circumstances in which the reach is not subject to direct intervention (i.e. whether geomorphic condition improves, remains relatively the same, or deteriorates). This analysis is framed in terms of the current trajectory of change under current management practices and predictions of recovery potential based over a 50-100 year timeframe. We recognise explicitly that future river responses/pathways will reflect management strategies. Hopefully, as strategies are implemented, the trajectory and potential for recovery will improve throughout the system. Hence, this assessment provides a contemporary statement of recovery pathways for river courses in Bega catchment. The procedures employed aid determination of the types of measures that can be undertaken to enhance the natural recovery processes of each river course. Adoption of these adaptive management principles requires responses to limiting factors and pressures that operate in the system. There are two steps in the determination of geomorphic recovery potential in Stage Three of the River Styles framework (Figure 6.1). Figure 6.1 Steps in Stage Three of the River Styles framework Stage Three, Step One: Determine the trajectory of river change ⇓ Stage Three, Step Two: Assess river recovery potential: Place reaches in their catchment context and assess limiting factors to recovery In the sections that follow, core products that appraise the geomorphic recovery of rivers across Bega catchment are presented. These include: Stage Three, Step One • Recovery diagrams for each River Style noting the trajectory of change of each reach. Stage Three, Step Two • Tables and flow diagrams that evaluate limiting factors to recovery operating in the catchment. 148 • Assessment of likely future pressures that may influence the system’s capacity for recovery. • A ‘catchment tree’ that outlines the linkage of geomorphic processes and their off-site impacts throughout the catchment. • A map showing the recovery potential of each reach within the catchment. 6.2 Stage Three, Step One: Determining the trajectory of river change Assessment of the trajectory of change for each reach of each River Style in Bega catchment was made using procedures outlined in Brierley and Fryirs (2005). The key is to determine where each reach sits on the recovery diagram, and evaluate its likely future trajectory of change. If suitable reaches are available, prediction of potential future target conditions entails space-time transformation (i.e. ergodic reasoning), where field sites that show differing stages of adjustment for the same type of river are used to make inferences about temporal patterns and rates of change. Where no examples existed, prediction was based on geomorphic insight, wherein an assessment is made of how the various geomorphic units that make up a reach are likely to adjust in the future, and how channel capacity and vegetation associations are likely to change. In Bega catchment, sensitive reaches have experienced irreversible change in River Style and now operate under altered catchment boundary conditions. These reaches are unlikely to regain their predisturbance condition. The range of potential recovery trajectories has been reduced and restoration is unachievable over 50-100 years. This scenario is evident for reaches of the Low sinuosity sand bed River Style and Channelised fill River Style, such that creation of a new condition is underway. Those River Styles that have experienced reversible geomorphic change but operate under altered catchment boundary conditions (i.e. rivers in confined and partly-confined valley-settings) have the potential to recover along either a restoration or creation trajectory. The trajectory taken depends largely on the geomorphic condition of the reach. The poorer the condition of the reach, the lower it sits on the degradation scale of the recovery diagram, and the less likely it is that restoration will occur. Reaches of the Partly-confined valley with bedrock-controlled discontinuous floodplain River Style and Confined valley with occasional floodplain pockets River Style fall into this category. Those reaches that have experienced reversible geomorphic change and operate under unaltered catchment boundary conditions either remain intact or sit on a restoration trajectory. In Bega catchment, these rivers are located in upland and escarpment zone settings (i.e. the Steep Headwater and Gorge River Styles), or are remnants of pre-disturbance geomorphic conditions (e.g. the Intact valley fill, Floodout and Low sinuosity boulder bed River Styles). 149 6.2.1 Trajectory of change for reaches of the Steep headwater, Gorge and Low sinuosity boulder bed River Styles All reaches of the Steep headwater, Gorge and Low sinuosity boulder bed River Styles are located in the most upstream reaches of river systems in Bega catchment. The catchment boundary conditions under which these River Styles operate have remained largely unaltered since European settlement. Many reaches of these River Styles are located in National Parks or Reserves. All reaches remain in good geomorphic condition. As these reaches retain their predisturbance structure and function, they are considered to be intact and sit at the top of the degradation pathway on the recovery diagram (Figure 3.11). To date, these reaches are yet to move into a degradation phase. 6.2.2 Trajectory of change for reaches of the Intact valley fill and Floodout River Styles Reaches of the Intact valley fill and Floodout River Styles are the last remaining remnants of discontinuous laterally-unconfined rivers in Bega catchment. The catchment boundary conditions under which they operate have remained largely unaltered following European settlement. All reaches are in good geomorphic condition and display pre-disturbance geomorphic structures. While vegetation associations on the fill surfaces have been altered, reaches are considered to be geomorphologically intact and sit at the top of the degradation pathway on the recovery diagram. They have not yet moved into a degradation phase. Concerted efforts are being made to restore the predisturbance vegetation associations of these River Styles. 6.2.3 Trajectory of change for reaches of the Confined valley with occasional floodplain pockets River Style The boundary conditions under which most reaches of this River Style operate have been altered following European settlement, inducing changes to vegetation, sediment and flow regimes. However, this River Style is resilient to change. As such, the pattern of morphological adjustment, and associated adoption of a restoration or creation pathway, depends on the extent of disturbance and subsequent degradation experienced by these reaches (Figure 6.2). No truly intact reaches remain in Bega catchment. However, reaches along Bemboka River, for example, have undergone minimal change. These reaches are considered to sit high on the degradation scale, approaching the turning point. With the re-emergence of pools and the improvement of vegetation associations, these reaches will move onto a restoration pathway with the potential to move back towards a predisturbance state. Those reaches that sit low on the degradation pathway have experienced considerable disturbance following 150 alterations to their catchment boundary conditions. Such reaches, including those along Wolumla Creek and lower Bega River, sit low on the degradation scale. They are characterised largely by sand sheets with (or without) exotic vegetation. Restoration is less likely along these reaches. Rather, creation towards a new geomorphic structure is the more likely pathway of future adjustment. Figure 6.2 Recovery diagram for the Confined valley with occasional floodplain pockets River Style Intact Restored Condition G A A B B G Turning Points Created E Condition E F D D F C C Degraded Good condition Moderate condition Poor condition reaches reaches reaches 151 6.2.4 Trajectory of change for reaches of the Partly-confined valley with bedrock controlled discontinuous floodplain River Style Like the Confined valley with occasional floodplain pockets River Style, reaches of this River Style operate under altered catchment boundary conditions. However, the River Style is inherently resilient to change. Hence, depending on the degree of disturbance, reaches may adjust along either creation or restoration pathways (Figure 6.3). In Bega catchment, most reaches of this River Style have experienced significant geomorphic change and sit lower on the degradation scale of the recovery diagram. Hence, restoration is unrealistic throughout the catchment. Geomorphic features such as islands and benches, which do not appear at any other stage in the evolutionary sequence, may develop. Channel capacity continues to adjust to the altered catchment boundary conditions. In Bega catchment, only one reach has adjusted towards restoration, along upper Tantawangalo Creek. With sustained recovery, it is likely that there will be a re-emergence of pools along this reach. Increased channel roughness will result in the retention of fine grained sediment and organic matter. Elsewhere in Bega catchment, reaches of this River Style remain on the degradation pathway. A few show initial signs of recovery, such as isolated reaches

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